1. Field of the Invention
The present invention relates to a novel material that can be applied to a light-emitting element. In addition, the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each using the material.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In the basic structure of light-emitting elements, a layer that includes a substance having a light-emitting property is interposed between a pair of electrodes. Voltage application to this element causes the substance having a light-emitting property to emit light.
Such light-emitting elements are self-luminous elements and have advantages over liquid crystal displays in having high pixel visibility and eliminating the need for backlights, for example; thus, light-emitting elements are thought to be suitable for flat panel display elements. Such light-emitting elements are also highly advantageous in that they can be thin and lightweight. Besides, very high speed response is also one of the features of such elements.
Furthermore, since such light-emitting elements can be formed in a film form, they make it possible to provide planar light emission easily; thus, planar light emission can be easily obtained with large-area elements using planar light emission. This is a difficult feature to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements using EL also have great potential as planar light sources applicable to lighting devices and the like.
Light-emitting elements using EL can be roughly classified according to whether the substance having a light-emitting property is an organic compound or an inorganic compound. In the case of an organic EL element in which an organic compound is used for the substance having a light-emitting property and a layer containing the organic compound having a light-emitting property is provided between a pair of electrodes, voltage application to the light-emitting element causes electrons and holes to be injected from a cathode and an anode, respectively, into the layer containing the organic compound, and current flows. The injection of both electrons and holes brings the organic compound having a light-emitting property into an excited state, and when the organic compound having a light-emitting property returns from the excited state to the ground state, light emission from the organic compound having a light-emitting property can be obtained.
Having such a mechanism, the above-described light-emitting element is referred to as a current-excitation light-emitting element. Note that an excited state formed by an organic compound can be a singlet excited state or a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
There are many problems in the improvement of element characteristics of such light-emitting elements which depend on substances, and element structure improvement, substance development, etc. have been carried out in order to solve these problems. For example, Patent Document 1 discloses a light-emitting element in which a compound having a stilbene skeleton is used for a light-emitting material, but the light-emitting element cannot be said to have sufficiently high reliability.
In addition to the process of light emission due to carrier recombination through current excitation, there is another process of light emission in which the excitation energy of an organic compound excited with current is transferred to another organic compound and accordingly the latter organic compound is excited to emit light. This process is effective in the case where emission efficiency is reduced due to stacking interaction caused by the high concentration of organic molecules that are desired to emit light (concentration quenching), and the process is generally applied to organic EL elements with an element structure in which a light-emitting material is dispersed in a light-emitting layer (a light-emitting layer is doped with a light-emitting material). Doping a host material with organic molecules that are desired to emit light suppresses the stacking interaction, so that the light-emitting element can have higher efficiency. In such a light-emitting element, excitation energy is transferred from a host material excited current to a dopant material, making the dopant material emit light. Note that when a substance A is dispersed in a matrix formed of a substance B, the substance B forming the matrix is called a host material while the substance A dispersed in the matrix is called a dopant material.
Among these dopant materials, types of materials that emit blue light are fewer than those of materials that emit light of a color having a long wavelength (e.g., red, orange, yellow, or green). One reason for this is that there is considered to be a limited selection of skeletons because a material for blue light emission needs to have small conjugation. Another reason is that the dopant material becomes easy to degrade due to energy for blue light emission which is higher than energy for light emission of a color having a long wavelength.
Therefore, a light-emitting element material for blue light emission is desired in order to provide a highly reliable organic EL element that emits favorable blue light.